In general, a method of caulking a boss is widely practiced as a method for fixing a metal plate etc. to a plastic part. In this method, a columnar protrusion (hereinafter referred to as a “boss”) of a proper size is provided at a predetermined position in a plastic part, while a hole into which the boss can be inserted is bored in a metal plate or a plastic plate to be fixed to the plastic part. After the two are fitted together, the boss protruding from the hole is subjected to ultrasonic vibration or heat to melt and deform and thereby caulk the boss.
The method using heat is today widely used because of its advantages over the method using ultrasonic vibration: (1) beautiful finish, (2) no influence of vibration on the part, (3) simple principle, hence easy maintenance, and (4) relatively low device cost compared with ultrasonic vibration. A heater used for caulking a boss is molded of a nichrome plate or formed by cutting a metal having a relatively high electrical resistance, and the heater uses Joule heat which is generated as a high current is passed therethrough.
One example of conventional thermal caulking devices is shown in FIG. 22. For the thermal caulking device shown in FIG. 22, a perforated metal plate 8′ is fitted with a boss 7a′ which is a columnar protrusion of a plastic part 7′. This thermal caulking device thermally caulks the boss 7a′ by pressing a metal tip 30, heated to or above the softening temperature of the plastic part 7′, against the leading end of the boss 7a′. 
As shown in FIG. 22, lead wires 31a, 31b are electrically connected by welding or screw fixation to flanges 30e, 30f, respectively, of the metal tip 30 which serves as a caulking heater. In a cavity 30c of the metal tip 30, a plastic cooling pipe 32, which delivers cooling air cooled to or below normal temperature, is disposed so that the opening end of the cooling pipe 32 is located near a pressing part 30a. The metal tip 30, the lead wires 31a, 31b, and the cooling pipe 32 are molded, for example, with epoxy resin 34, so as to retain their respective desired positions.
FIG. 23 is an external perspective view of the metal tip 30 as a single part. The metal tip 30 shown in FIG. 23 has the pressing part 30a which has an upwardly convex spherical surface, and a cylindrical wall part 30b rises from around the pressing part 30a, forming the cavity 30c inside the metal tip 30. The wall part 30b is divided by slits 30s into left and right portions, and the flanges 30e, 30f are formed at the upper end of the wall part 30b. 
In FIG. 22, the pressing part 30a of the metal tip 30 is depicted at a position above and at a distance from the boss 7a′. When thermal caulking work is performed, a current flows from the lead wires 31a, 31b to the metal tip 30, and the metal tip 30 is heated by Joule heat. Upon reaching a predetermined temperature, the metal tip 30 is lowered as indicated by the arrow Y to bring the pressing part 30a into contact with the leading end of the boss 7a′. Then, the leading end of the boss 7a′ is softened and melted by the heat of the metal tip 30. Thereafter, the application of a current to the lead wires 31a, 31b, is stopped, and cooling air is sprayed from the cooling pipe 32 to the pressing part 30a of the metal tip. The cooling air flows through the slits 30s to the outside. The metal tip 30 and the leading end of the boss 7a′ are cooled to a temperature equal to or lower than the softening point of plastic to solidify the leading end of the boss 7a′. Then, the metal tip 30 is raised again, which completes the caulking work.
FIG. 24A and FIG. 24B are cross-sectional view showing a state at the start of thermal caulking work and a state at the end of the thermal caulking work. FIG. 24A shows a stage in which a current is applied from the lead wire 31a to the lead wire 31b to heat the metal tip 30, and the pressing part 30a is pressed against the leading end of the boss 7a′ to melt and deform the boss 7a′. Specifically, in FIG. 24A, electrical power is supplied from a power source (not shown) to the lead wire 32a, and a current flows from the lead wire 31a to the flange 30e, the wall part 30b, the pressing part 30a, the wall part 30b, and the flange 30f of the metal tip 30, and the lead wire 31b. Thus, the flange 30e, the wall part 30b, the pressing part 30a, the wall part 30b, and the flange 30f are heated in this order according to the flow of the current. When the pressing part 30a reaches a temperature exceeding the softening point of plastic, deformation of the leading end of the boss 7a′, i.e., caulking work starts. By being pressed with the pressing part 30a, the leading end of the boss 7a′ assumes a predetermined shape.
FIG. 24B shows a state in which, after the leading end of the boss 7a′ has been melted and deformed, the application of a current from the lead wire 31a to the lead wire 31b is stopped, and coaling air is sprayed from cooling air delivery means (not shown) through the cooling pipe 32 to the pressing part 30a inside the cavity 30c to discharge the heat of the metal tip 30 and the melted boss 7a′ through the slits 30s and thereby cool the metal tip 30 and the boss 7a′. The flow of the cooling air inside the cavity 30c of the metal tip 30 is indicated by the arrows in FIG. 24B. The cooling air collides with the pressing part 30a inside the cavity 30c, rises along the wall part 30b, and flows through the slits 30s to the outside of the metal tip 30. That is, the heat of the metal tip 30 and the melted boss 7a′ is discharged through the slits 30s. Thus, the boss 7a′ of the plastic part 7′, which is a molded part, is melted, deformed, cooled, and solidified, so that the perforated metal plate 8 is fixed to the plastic part 7′ (e.g., see Patent Document 1).
FIG. 25 is a schematic cross-sectional view of another conventional plastic part thermal caulking device which uses high-frequency induction heating means as metal tip heating means. In the thermal caulking device shown in FIG. 25, a plurality of metal tips 40, each having a cavity 60 through which a cooling fluid is circulated, are respectively attached under cooling pipes 70, and an induction heating coil 51 is wound on the outer periphery of each metal tip 40. A high-frequency induction power source 50 is used to pass a high-frequency current through the coil 51 and generate an induced current in the metal tip 40 to thereby heat the metal tip 40. Then, a pressing part 45 of the headed metal tip 40 is pressed against the leading end of the boss 7a′ to melt and deform the leading end of the boss 7a′. After the leading end of the boss 7a′ has been melted and deformed, the application of a high-frequency current to the coil 51 is a stopped, and a cooling fluid, such as cooling air, is sprayed from the cooling pipe 70 inside the cavity 60 of the metal tip 40 in the direction toward the pressing part 45, to cool the metal tip 40 and the heated and melted boss 7a′. Thus, the boss 7a′ of the plastic part 7′, which is a molded part, is melted, deformed, cooled, and solidified, so that the perforated metal plate (object to be fixed) 8′ is fixed to the plastic part 7′. Since high-frequency induction heating can instantly heat the metal tip 40, it has an advantage over electrical heating in that the caulking step takes a shorter time (e.g., see Patent Document 2).
Thermal caulking of plastic parts is used not only for caulking bosses and shaft parts of about several millimeters in diameter, such as thermally caulking the rotation shaft of plastic scissors of daily use, but also for thermally caulking smaller plastic parts. For example, in the case of a forceps-type electrical treatment tool shown in FIG. 26, a first forceps piece 12b and a second forceps piece 14b are incorporated into a support 67 mounted at the leading end of a catheter tube 47 which can be inserted into the body, and the first forceps piece 12b and the second forceps piece 14b are supported on a plastic support shaft 80. The support shaft 80 is thermally caulked after the support 67, the first forceps piece 12b, and the second forceps piece 14b are passed therethrough, and thus the first forceps piece 12b and the second forceps piece 14b are assembled so as to be able to open and close (e.g., see Patent Document 3).
FIG. 27 is a schematic view showing a step in which heated rod-like metal tips 110 are pressed respectively against both ends of the support shaft 80 of the support 67 mounted at the leading end of the catheter tube 47 to thermally caulk the support shaft 80. The catheter tube 47 is intended to be inserted into the body, such as a blood vessel, and the support 67 as well as the support shaft 80 counted at the leading end of the catheter tube 47 are also small parts. During operation, the first forceps piece 12b and the second forceps piece 14b at the leading end of the catheter tube 47 inserted into the body are required to open and close as intended by a surgeon (doctor). It is undesirable that, the first forceps piece 12b and the second forceps piece 14b do not move smoothly due to too tight caulking, or that the first forceps piece 12b and the second forceps piece 14b come off due to too loose caulking.
Other than the above example, thermal caulking of a stent which is put inside the body by being mounted at the leading end of a balloon catheter, and thermal caulking of a plastic part at the leading end of an endoscope are also known as examples of thermal caulking of catheter-related plastic parts.
FIG. 28A shows an extended state of a stent 17 which is put inside the body by being mounted at the leading end of a balloon catheter. FIG. 28B shows a state in which the stent 17 shown in FIG. 28A is formed into a ring shape and a caulking ring 18 is put thereon. FIG. 28C is a schematic view showing a step in which the caulking ring 18 shown in FIG. 28B is pressed from both sides in the vertical direction with a pair of heated metal tips 120 to thermally caulk the caulking ring 18 (e.g., see Parent Document 4),
FIG. 29 shows an endoscope attachment of which a fixing member 41 is fixed by press-fitting and thermal caulking inside a tube 37 at a leading end part of an endoscope (e.g., see Patent Document 5).
Thus, for thermal caulking of small plastic parts, especially medical plastic parts, it is required that the thermal caulking device itself strictly manages heating and pressing conditions of the metal tip and performs thermal caulking stably and precisely under the required best heating and pressing conditions.